Direct correlation of SPM (Scanning Probe Microscopy) techniques with
Raman scattering was a dream…now its a reality. The combined Nanonics MultiView/Renishaw inVia Raman Microscope systems are the first systems to allow true on-line SPM with Raman imaging

Features:

• Combined on-line pixel by pixel surface topography and chemical characterization via Raman spectroscopy at ultrahigh resolution.
• Tip enhancement Raman spectroscopy (TERS) with metallic coated AFM tips.
• AFM and Raman spectroscopy of MEMS devices. High resolution of local silicon stress in MEMS devices.
• Fully integrated with upright and inverted microscopes through the use of transparent cantilevered bent tips and a 3D Flat scanner.
• Significant resolution improvement due to topography-corrected focus through height adjustment via online AFM feedback
• Hardware and software solutions for Raman correlated mapping through “collage” maps, intensity maps, band position maps, measurement and analysis of correlated near field and far field spectra (2 points measurement), tip and sample scanning combinations.
• Multiple probe option of Raman interfered probe (tip enhancement, nano indentation, force regulation and high resolution AFM probe.

Unique Applications

The image (left) is a combined AFM topographic image with Raman map of a graphite-diamond film. The 3D surface shows the topography variations and the colours indicate the Raman Intensity of the 1331cm^-1 band. Raman spectra have been collected simultaneously with the topographic data through an AFM scan.  Surface theory of this film predicts that the diamond grow in the topographical valleys. The collage image shows higher intensities of the diamond band (1331cm^-1) in the valleys as predicted.

SPM and Raman Integration

The hardware and the software of the Nanonics NSOM/AFM MultiView systems are transparently integrated with a Renishaw Raman Microscope

There are two major factors that enable the Nanonics AFM system to be successfully integrated with Raman spectroscopy:

1) Nanonics' patented, transparent, glass cantilevered probes
2) Nanonics' flat and ultrathin scanning platform that makes use of four piezo drivers. These cylindrical piezos are placed in a quadrant fashion around the sacnner.

Nanonics' Unique Probes

Probe geometry: Nanonics systems use patented cantilevered optical fiber probes. The cantilevered optical fiber is held between the objective lens and the sample without obstructing any aspect of the far-field conventional upright or inverted microscope. The tip of the fiber is exposed, allowing direct viewing of the scanned region. The probe geometry and dimensions can be easily customised for different applications

Advantage over standard silicon probes: These probes overcome the problem of obscuring the far filed view of the upright microscope. Standard silicon tips have a flat cantilever which blocks the field of view from an upright microscope Does not generate any background Raman signal unlike regular silicon probe. Easily customized to different samples and applications Taking advantage of the points above, There are NO LIMITATIONS on Samples, both opaque and transparent samples can be used with out any interference with the Raman data

3D FlatScanner Platform

Flat Scanner: Nanonics SPM platforms are optically and micro-Raman friendly. The platforms make use of four cylindrical piezos placed around the platform in a quadrant fashion (as shown in Fig.4). As opposed to upright-architectured piezos used in conventional SPM (see fig.?) platforms, the Nanonics scanning stage is very thin (7mm) allowing the head to be integrated into all upright optical microscopes. The open optical axis allows for free viewing of >20mm.The platform also allows for rough scanning whereby the sample under inspection can be rapidly moved via inertial motion mechanism.

 

Advantages of the flat scanner: Free optical axis for upright and inverted optical microscope view. Scanning small and large topography due to the flexible scan ranges from nanometer scale to approximately 100 microns in XY and up to 70 microns in Z. Sample fine positioning through inertial motion in increments of 1 micron up to 2-3mm.

Topographical and Chemical Characterisation- Data Correlation

For achieving AFM Raman combined data, the tip is adjusted to the center of the far field Raman illumination laser spot, the area of interest on the sample is brought to that point (by inertial motion), AFM parameters are defined, points for Raman measurement are defined relative to the AFM scan parameters, Raman settings are selected, running the scan, AFM imaging in progress, Raman spectra are acquired within the defined pixels during the AFM scan, scan is completed.
Raman map images can be created in different types such as mapping the intensity of a specific band at the different points or mapping a band shift in wave numbers, etc…
Raman maps can be merged with the topographical data to perform collage images.

Collage images of the diamond films surface with Raman intensity of the 1331 cm^-1 band (a) and 1525 cm^-1 band.

Such collage images are presented in fig.8 by a 3D presentation of the data of a diamond film. The surface describes the topography variation and the colures indicate the intensity of the 1331  cm^-1 band (fig. 8a) and the 1525 cm^-1 band (fig. 8b). It is clearly seen that the combined data shows the materials composition within the topographic image and the direct location of each band. The green arrows show the Carbon (1525 cm^-1 band) locations on the surface when the 1525 cm^-1 band intensity is mapped.

Software flexibility of generating different Raman maps.

Beside the intensity map, the system offers variety of mapping options such as mapping a different bands within the range of collected spectra, mapping wave number shift through curve fitting option, mapping relative intensity of two peaks, etc. 
 

Figure 9 demonstrate this flexibility of creating different maps of an indentated silicon feature shown in (a) and (d) as an 2D and 3D AFM presentation respectively. (b) and ( c) demonstrate two Intensity Raman maps of the silicon 520 cm^-1 band and the stressed silicon 523.7 cm^-1 band. Curve fitting procedure has been carried out on the peak in the range of 510 cm^-1 and 530 cm^-1 and peak position (wave number position) was obtain in (e).

Beside the correlated data material composition of the scanned area and the topographical image, one can investigate the stress and strain fields in these samples. Advantages of such applications can be taken into the filed on MEMS devices as will be shown later.

Line profiles with 170nm lateral Raman resolution

For some application, correlated AFM Raman maps are not necessary and line profiling can add more light to the picture. Figure 10 shows a combined overlaid point by point AFM Raman line profiles of a line scan on a TiCN sample.
Three regions are interesting in this plot. In Region 1 the AFM is changing by ~0.4 um while the Raman is rigidly constant (see the blue arrow in Region 1) since the material composition is not changing.In Region 2 small AFM change and material change is clearly seen. The Raman lateral change occurs on a lateral scale of 170nm (distance of two points is 85nm).  In Region 3 the AFM is essentially constant while the Raman, which represents the material composition is changing quite drastically.

Lateral resolution improvement of Raman by on-line AFM

In confocal microscopy such as Raman spectroscopy, the lateral resolution is limited by the diffraction limit. For achieving such resolution out-of-focus light should be eliminated. Pin hole is used to reduce the out of focus on the detector side during the confocal scan. However, this do not solve the problem of the out-of-focus light by illuminating the surface due to the topography variation of the sample, see fig. 11.

The combination of on-line AFM Raman keeps the investigated point of the sample at a constant distance from the microscope objective defined by the tip Z position and controlled by the AFM feedback. The AFM feedback loop of the tip with the sample works as a Z control to adjust tehe sample height to be in focus all along the scan. Hence, the illumination and the collection from the sample are achieved with considerable reduce of the out-of-focus light. See fig. 12.

AFM Raman on MEMS devices

High resolution local silicon stress on MEMS structures

One of the major advantages of having an on-line AFM is the ability to use the probe for mechanical manipulation on a nano-metric level.  On-line AFM can impose finely controlled and well-defined strain on a sample with high pressures.  Since P=F/A, the smaller the tip, the higher the pressure.  Since the area of the probe (A) is nano-metric the strain can reach up to megapascals of pressure.  NanoRaman technology is a new and ideal solution for super-resolution silicon stress measurements in floating structures such as combs and forks or MEMs devices (Fig.13a).   In Figure 13b, we see an AFM probe applying defined forces to a floating MEMs cantilever.  At the same time the on-line Raman measures the shift in the silicon vibrational frequency and silicon strain at the point defined by the cross.  In Figure 13c, we see the shift in the Raman signal as a function of the applied stress.  To date, no other system is capable of such a combination of on-line mechanical nano-metric manipulation and chemical characterization as well as the imposition of nano-electrical perturbations with Raman scattering.

AFM Raman characterizations on silicon and strained silicon on transistor device

When silicon is deposited on top of a different substrate where the atoms are spaced farther apart, the atoms in silicon stretch in order to line up with the atoms of the substrate beneath, “stretching” -- or "straining" -- the silicon. In the strained silicon, electrons experience less resistance and flow up to 70 percent faster, which can lead to microchips that are up to 35 percent faster -- without having to shrink the size of the transistors used.
Using AFM investigating of such devices silicon and strained silicon will not be distinguishable. The combination of AFM and Raman mapping can perform both types of data topography and stress/strain characterizations.
Figure 14 demonstrate an AFM combined Raman maps of silicon and strained silicon device. 2D topography image is presented on figure 14(a). Figures 14(b) and (c ) demonstrate the Raman Intensity if the 520 cm^-1 silicon band and the 503 cm^-1 strained band correspondently. These two maps clearly show the location of the strained silicon on this chip. For example, the two features coming from the right side of the AFM image look exactly the same in terms of height as the other part of the same image. However, the Raman maps on (c ) shows that these two features are infact strained silicon.  (d) and (e) are collage images combining the surface variations and the Raman intensities of the siliconand th strained silicon band. 

Tip Enhancement Raman Spectroscopy (TERS)

A further advantage to on-line AFM and Raman systems is that Near-filed techniques can be readily applied through the use of NSOM probes. The Raman signal is intrinsically weak (less than 1 in 107 photons) and the laser power emerging from tip is extremely low (typically 100 nW) because of the low optical throughput of metal-coated fibre tips. The long integration time (typically 10 minutes per spectrum) required for collecting good quality Raman spectra makes it impractical to construct a Raman image through this conventional method.
 However, Near-field scanning optical microscopy (NSOM) has been successfully integrated with Raman spectrometry using an apertureless configuration, in which the laser is focused onto the sample through a microscope objective and Raman signal is collected by the same objective. This is similar to the conventional micro-Raman except that a metal tip is brought into the laser spot on sample surface to enhance the Raman signal through surface enhanced Raman scattering (SERS), allowing Raman enhancement by several orders of magnitude (≈104) and Raman mapping on real silicon devices with a1 second exposure time

The Data in Figure 15, was obtained using Nanonics’ specialised surface enhanced Raman Scattering probes on a sample of Silicon with a 70-90nm strained layer of Silicon. In this case the data was recorded using the MultiView 2000 System, which allows probe movement independent of sample scanning. This allows one to first position the tip relative to the Raman laser, at a point where maximum enhancementof the Raman scatter is obtained and then, keeping the tip position constant with nanometric precision. Three spectra were recorded, near filed enhanced spectrum (a) far-field spectrum and the difference spectrum (c ). The near field signal is collected while the tip is in contact with the surface and then the tip is retracted to a certain distance (by the upper piezo scanner of the MV2000 head) with out changing the position of the sample and far field signal is collected. The software substrates these two signals giving the difference signal.